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Posted: March 14, 2007

Microbes survive in soil with fullerenes

(Nanowerk News) Natural settings may diminish the high toxicity of fullerenes or Cv60 nanoparticles to soil-dwelling microbes, according to new results published today on Environmental Science & Technology's Research ASAP website ("Impact of Fullerene (C60) on a Soil Microbial Community "). The findings contradict previous studies, which have shown that the nanoparticles are lethal in more unsoiled settings.

The new experiments, conducted under some of the most realistic conditions yet with microbes in soil, come from a team led by Ron Turco of Purdue University. The researchers exposed soil microbes to relatively high concentrations of C60 (1000 parts per million) both in dry form and in an aqueous suspension with tetrahydrofuran. They also exposed the microbes to tetrahydrofuran alone. This solvent is typically used in lab experiments to suspend C60 because the particles are not soluble in water at room temperature. The team incubated their C60 concoctions in real soil samples taken from corn plots northwest of the Purdue campus.

While exposing soil to the C60 nanoparticles over several months, Turco and co-workers measured the presence and activity of microbial populations. To see how the bugs were faring, the researchers assessed glucose respiration, microbial biomass, enzymatic activities, and DNA extracts, among other measures of the populations’ health.

Overall, the team found little effect on the structure or function of the soil microbial community. Because microbes are at the bottom of the food chain, Turco points out, effects on them have the potential to work their way up to higher trophic levels. But so far, “there is no effect on that key part of the food chain,” he says. “Is there a possible effect that we don’t see? Yeah, there could be,” he continues. Those effects could come from dosing effects or different possible arrangements of the fullerenes themselves, such as various outer coatings that manufacturers apply to obtain certain functions.

Coated nanomaterials will interact with more complex environments differently, with regard to both exposure and potential for mobility. Organic matter, for example, might make some nanomaterials move farther while sequestering others, depending on their chemistry. Also, “you cannot extend this [new research] to other types of nanomaterials,” such as titanium nanoparticles, says Mark Wiesner of Duke University. However, “this is a really important study because there has been, based on really preliminary and methodologically young studies, a lot of very strong reaction to the potential for nanomaterial toxicity,” he adds.

The new research includes factors missing from previous studies, with a soil matrix that “resembles much more closely a real exposure setting,” Wiesner continues. “That there is not a significant impact on the microbial community is a very different result than has been found in these more pristine systems studied.”

Delina Lyon, a graduate student who studies fullerenes and bacteria at Rice University, notes that the field is young enough that researchers still “don’t know what a relevant concentration is to use” when testing antibacterial activity. In past studies, bacteria have been directly exposed to fullerenes, but that exposure can be mediated if organic matter causes the nanoparticles to cluster or adsorb to soil. “It depends also on what matrix you use—maybe in soil it’s not going to have an impact, but maybe in a lake,” stream, or other open water system, she says, “lower fullerene concentrations might be relevant.”

Long-term groundwater experiments also could be particularly interesting, Lyon suggests, to determine how water might carry nanoparticles through soil. She adds that in soil, it’s also possible that fungi or bacteria could chemically alter nanoparticle surfaces and thereby make them more mobile. Lyon says that she was pleased to see the variety of methods that Turco and his co-workers used to assess toxicity.

“I don’t know of any other study to date [regarding potential nanomaterial impacts] that has taken such a holistic approach with quantitative outcomes,” agrees John Fortner, who is a postdoctoral researcher at the Georgia Institute of Technology, where he focuses on the fate and transport of nanomaterials. Soil biology is complicated, he notes. With billions of bacteria living in 1 gram of soil, studying bioavailability is complex. “But you have to start somewhere,” he says.